Magnetic plates or chucks for retention of a magnetically attractable workpiece, e.g. an iron or steel workpiece, are commonly used in surface grinding and other machines in which a vise-like or clamp-type gripping of the workpiece is undesirable because the jaws of the vise, clamp or the like would interfere with the machining operation or difficulties may be encountered in engaging or disengaging the vise-like structures.
Such magnetic plates have been of three basic types heretofore. For example, they may be provided exclusively with permanent magnets which are mechanically moved toward and away from a mounting plate on which the workpiece is disposed. The disadvantage of this system, of course, is that considerable force is necessary to break the magnetic attraction with which the permanent magnets retain the workpiece. Thus the operation of this type of device, its construction and its adjustment can be rather complicated.
In still another arrangement, a pole plate is provided at the top of the magnetic holder, a sole plate of soft magnetic material is provided as the base and a housing is disposed between the pole plate and the sole plate and encloses an array of fixed-magnetic-sense permanent magnets and variable-magnetic-sense electromagnets, i.e. so-called electropermanent magnets which retain the magnetic sense in which they are set by the application of an electrical pulse through a coil surrounding these magnets. Magnetic holders of this type are the basis for the present improvement.
Mention may also be made of electromagnetic holders in which the electromagnetic force is applied by passing an electric current through the coils surrounding magnetic cores. As long as these coils remain energized, the magnetic retention field is applied. In order to disengage the workpiece, the electrical energy is disconnected and the magnets are de-energized. The disadvantage of the latter system, of course, is that a large amount of energy is required to maintain the magnetic force.
As noted above, the invention is concerned primarily with magnetic holders using magnets of the permanent and electropermanent type, since these holders have the advantage that the energized state may be established by the application of an electrical pulse to retain the workpiece by the magnetic forces of permanent magnets and without further electrical energization. The application of a reverse pulse can, however, nullify the magnetic field to permit the workpiece to be removed from the magnetic holder.
For the most part, such magnetic holders comprise permanent magnets of invariable magnetic field direction composed of a material having a high coercive force and reversible-direction magnets, which are set by pulses through respective coils, of relatively low coercive force. The magnets are disposed so that there is no danger of demagnetization of the high-coercive-force magnets by the passage of the pulses through the coils.
In one conventional system of this type, the space between two pole pieces of a magnetic core is occupied by a central magnet of the electropermanent type juxtaposed with two permanent magnets. While the magnetization of the electropermanent magnet is in the same sense as that of the two permanent magnets, the two pole pieces adjacent them become respectively North and South poles, corresponding to the operative condition of the holder. By reversing the direction of magnetization of the central magnet, the lines of force between the two permanent magnets can close through the central magnet and thus prevent any magnetic field from bridging the poles externally of the system. This corresponds to the inoperative state of the holder.
While the system just described functions effectively, it has certain disadvantages. For example, it is of complicated construction and requires careful adjustment. More particularly, it is necessary to provide sets of three magnets between two pole pieces in such spacing that air gaps are avoided between the magnets and the pole pieces. This means careful adjustment during assembly which is a particular inconvenience.